This invention relates to an electric motor assisted turbocharger, and in particular to a turbocharger for an internal combustion engine.
Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing.
In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted, an annular inlet passageway arranged around the turbine chamber, an inlet arranged around the inlet passageway, and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine chamber and rotates the turbine wheel.
Under steady state conditions of engine speed and load a conventional turbocharger can supply the required amount of air to the engine for efficient combustion. However, there are other conditions, such as at engine start up or transient conditions such as a sudden requirement for a high load from the engine, in which the energy in the exhaust gas is not sufficient to enable the turbocharger to deliver the required air supply to the engine quickly enough. Modern engines are designed to reduce engine fuelling in such circumstances to avoid high levels of exhaust emissions through incomplete combustion, and accordingly engine response suffers during such transient or other conditions.
It is known to address the above problem by providing a turbocharger with an integral electric motor to assist rotation of the compressor to improve the response of the turbocharger and thus the engine performance. An example of such an electric motor assisted turbocharger is disclosed in U.S. Pat. No. 5,604,045. The electric motor is essentially a synchronous motor located within the turbocharger bearing housing, and comprising a magnetic rotor assembly mounted to the turbocharger shaft surrounded by a fixed stator comprising field coils wound on magnetically permeable pole pieces. The operation of the synchronous motor is essentially conventional in that the field coils are energised with an AC supply to create a rotating magnetic field around the shaft which couples with the magnetic field of the magnetic rotor. The motor may be energised whenever the turbocharger requires power assistance to ensure optimum air supply to the engine.
With the above known form of electric motor assisted turbocharger, a synchronous motor comprising a magnetic rotor is used to avoid the need for commutation. A disadvantage of the synchronous motor is that relatively complicated control electronics are required as the excitation frequency of the stator coils must always be matched to the rotational speed of the turbocharger, so that both a variable frequency control signal and means for monitoring the speed of the turbocharger are required. See for instance the control system disclosed in PCT patent application WO98/16728.